I would buy new also because I my experience, usually when they are past a certain age it's not worth rebuilding, we get it running again some kind of way but it's one problem after the next, don't ask me why, you might get another year out of it but they're never the same, yours having that sentimental value tends to support what I just said, if that helps any, hope you can let it go... You could if you have spare cash, rebuild the old as a backup but chances are you'll still run into what I spoke of...
It will serve again once or twice like in the old days, you get that feeling back but the rest of the time it sits around collecting dust...
Or you try and put it into service again but it doesn't cut it like it used to, not sure how to explain it but they're never quite the machine you used to have.

Some overheat regularly, others can't get up to Wot right, still some vibrate endlessly, others still the new engine shakes other parts to breaking, the rest don't even want to run, and it's always something.